1. Field of the Invention
This invention relates to a method for the production of a semiconductor.
2. Description of the Prior Art
It was found in 1975 by W. E. Spears et al. that on the amorphous silicon obtained by the plasma decomposition of silane (SiH.sub.4), valence electron control can be effected by doping the amorphous silicon with phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6). In 1976, D. E. Carlson et al. succeeded in trial manufacture of a solar cell using amorphous silicon. The amorphous silicon thus leaped in the limelight of scientific concern and has since been inducing active researches devoted to exploring the feasibility of the substance as the material for solar cell.
As fruits of such researches conducted on the pin heterojunction type solar cell, since A. Catarano et al. unveiled their success in attaining a conversion efficiency of 10.1%, several research results have been published indicating conversion efficiencies exceeding the level of 10%. Consequently, the pin hetero-junction type solar cell has come to attract growing interest as a thin-film solar cell of high-conversion efficiency.
The amorphous silicon films used in forming solar cells of high conversion efficiency are invariably produced by the glow discharge decomposition using power sources of frequencies near or above the level of 13.56 MHz. Successful production of a solar cell of such high conversion efficiency by the glow discharge decomposition using a power of a lower frequency has not yet been reported. The necessity for this high frequency may be logically explained by a postulate that when the parallel plate type electrode which has an electrode disposed parallel to a substrate and a susceptor and effects glow discharge preponderantly between the electrode and the substrate or the susceptor is used to effect the glow discharge under conditions involving a low frequency, a solar cell of high performance is not obtained because the time required for field inversion is long, the energy given to various ions produced by the glow discharge decomposition is large in magnitude, and the semiconductor film being deposited on the substrate suffers from degradation of quality because of collision of such ions against the film. Particularly in the discharge at a frequency of less than 13.56 MHz which takes up much time in field inversion, the degradation of the film quality is conspicuous because the kinetic energy possessed by ions is large.
On the other hand, when the semiconductor film is produced by using a power source of a frequency exceeding about 13.56 MHz, there are inevitably entailed the following problems.
(1) Necessity for impedance matching is inevitable and consequent complication of equipment is unavoidable. PA1 (2) Noise trouble in the megahertz zone which impedes automation of equipment is liable to occur. PA1 (3) Power distribution within the electrode has no insignificant effect when the semiconductor film to be produced has a large surface area.
It is considered that these problems may be precluded or notably mitigated by lowering the frequency of the power source. When the frequency of the power source is lowered, however, initiation of discharge becomes difficult particularly in the case of DC glow discharge. This drawback is not very serious in the case of an internal electrode. In the case of an external electrode such as a glass tube or a quartz tube which has an exposed electrode, it is said that discharge under ordinary conditions is difficult to obtain unless high voltage is applied or a trigger is provided or some other remedial measure is taken.
In view of the true state of affairs described above, the inventors continued a diligent study in search of a method capable of producing a semiconductor of high performance even by the use of a power source of a low frequency. They have consequently perfected this invention.